A reflection after an act; a later or subsequent thought, action, or expedient.

"She packed her sandals only as an afterthought, but she was glad she did."

Something additional to the original plan or concept.

"This whole remodeling was an afterthought; we were only going to paint and call it good."

(very rare except in the past tense, possibly nonstandard) To expound as an afterthought.

To think about after the fact, reflect on.

To repent, or feel remorse for; to regret.